Researchers from University of California, Los Angeles (UCLA) studied sleeping patterns among traditional people - the Hadza of Tanzania, the San of Namibia and the Tsimane of Bolivia - whose lifestyles closely resemble those of our evolutionary ancestors.

"The argument has always been that modern life has reduced our sleep time below the amount our ancestors got, but our data indicates that this is a myth," said Jerome Siegel, leader of the research team and professor of UCLA's Semel Institute of Neuroscience and Human Behaviour.

The researchers collected sleep records on 94 adults from among the Hadza, hunter-gatherers who live near the Serengeti National Park, the Tsimane, hunter-horticulturalists who live along the Andean foothills and the San hunter-gatherers in the Kalahari Desert for a total of 1,165 days.

One myth dispelled by the results is that in earlier eras people went to bed at sundown. The subjects of the study stayed awake an average of 3 hours and 20 minutes after sunset, researchers said.

"The fact that we all stay up hours after sunset is absolutely normal and does not appear to be a new development, although electric lights may have further extended this natural waking period," said Siegel, who is also chief of neurobiology research at the Veteran Affairs of Greater Los Angeles Health Care System.

Most of the people studied slept less than seven hours each night, clocking an average of six hours and 25 minutes.

The amount they slept varied with the seasons, with the study's subjects averaging six hours in the summer and just under seven hours in the winter. Still, they rarely took naps.